Stents are generally cylindrical shaped devices that are radially expandable to hold open a segment of a blood vessel or other anatomical lumen after implantation into the body lumen. Stents have been developed with coatings to deliver drugs or other therapeutic agents.
Stents are used in conjunction with balloon catheters in a variety of medical therapeutic applications including intravascular angioplasty. For example, a balloon catheter device is inflated during PTCA (percutaneous transluminal coronary angioplasty) to dilate a stenotic blood vessel. The stenosis may be the result of a lesion such as a plaque or thrombus. After inflation, the pressurized balloon exerts a compressive force on the lesion thereby increasing the inner diameter of the affected vessel. The increased interior vessel diameter facilitates improved blood flow. Soon after the procedure, however, a significant proportion of treated vessels re-narrow.
To prevent restenosis, short flexible cylinders, or stents, constructed of metal or various polymers are implanted within the vessel to maintain lumen size. The stents acts as a scaffold to support the lumen in an open position. Various configurations of stents include a cylindrical tube defined by a mesh, interconnected stents or like segments. Some exemplary stents are disclosed in U.S. Pat. No. 5,292,331 to Boneau, U.S. Pat. No. 6,090,127 to Globerman, U.S. Pat. No. 5,133,732 to Wiktor, U.S. Pat. No. 4,739,762 to Palmaz and U.S. Pat. No. 5,421,955 to Lau. Balloon-expandable stents are mounted on a collapsed balloon at a diameter smaller than when the stents are deployed. Stents can also be self-expanding, growing to a final diameter when deployed without mechanical assistance from a balloon or like device.
Stents can currently be made of nitinol, which is a nickel titanium alloy. The shape memory and super elastic properties of nitinol are useful in medical devices. Unfortunately, nitinol is difficult to weld because nitinol forms an oxide layer on its surface that makes it difficult for the melt of the weld pool to reach the base metal during the welding process and achieve a good weld. Further, nitinol welds tend to be brittle, increasing the chance of weld failure. Welding difficulties can increase stent manufacturing costs due to increased manufacturing time and added quality control requirements. Poor welds can also decrease the performance of stents in use should the welds fail after implantation in the patient.
Other problems exist with drug eluting stents, which currently employ exterior coatings with or without polymers on metal struts to hold a drug for subsequent elution and delivery of the drug to surrounding tissue. Unfortunately, the coatings can be fragile and can fracture and fragment during manufacture, delivery, deployment, or use. Fracture during manufacture increases the cost and complexity of manufacture. Fracture during delivery, deployment, or use can reduce the effectiveness of the stent due to lost drug and can pose a risk to the patient if fragments block blood flow. The drug elutes from the coating surface, so the duration of drug elution is limited by the coating thickness, i.e., the mean diffusion length of the drug within the polymer coating. Concerns have also been raised over the long-term effects of polymers in contact with the body.
It would be desirable to have a welded stent and stent delivery system that would overcome the above disadvantages.